Investigations on the reaction mechanism of the skeletal

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Applied Catalysis A: General 206 (2001) 57–66
Investigations on the reaction mechanism of the skeletal
isomerization of n-butenes to isobutene
Part I. Reaction mechanism on H-ZSM-5 zeolites
D. Rutenbeck a , H. Papp a,∗ , D. Freude b , W. Schwieger c
a Universität Leipzig, Institut für Technische Chemie, Linnéstr. 3, 04103 Leipzig, Germany
Universität Leipzig, Institut für Experimentelle Physik I, Linnéstr. 5, 04103 Leipzig, Germany
Friedrich-Alexander-Universität Erlangen-Nürnberg, Lehrstuhl für Technische Chemie I, Egerlandstr. 3, 91058 Erlangen, Germany
b
c
Received 22 May 1999; received in revised form 18 April 2000; accepted 18 April 2000
Abstract
A series of H-ZSM-5 zeolites with Si/Al ratios between 17 and 170 were tested in the skeletal isomerization of n-butenes
to isobutene in the temperature range of 573–773 K. A selectivity to isobutene of ca. 20% was obtained with all zeolites at
573 K independent of the conversion of n-butenes. With increasing temperature the conversion of n-butenes increased on the
zeolites with Si/Al ratios up to 70 (high number of acid sites), whereas the yield of isobutene and the selectivity to isobutene
decreased. On the zeolites with higher Si/Al ratios (160 and 170) an opposite behaviour was observed; the conversion of
n-butenes decreased while the yield of isobutene and the selectivity to isobutene increased with increasing temperature. A
selectivity to isobutene of nearly 90% could be obtained with the latter zeolites at 773 K. These observations are interpreted
in such a way that on H-ZSM-5 zeolites with a high number of acid sites, isobutene is formed via the bimolecular mechanism
independent of the reaction temperature. On H-ZSM-5 zeolites with a low number of acid sites, a change from the bimolecular
to the monomolecular mechanism takes place with increasing temperature. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Skeletal isomerization; Isobutene; ZSM-5; Reaction mechanism
1. Introduction
Stringent limits for air pollutants enacted in
many countries have led to an increased demand
for isobutene, since this compound is used for the
production of methyl tert-butyl ether (MTBE), an
oxygenate with a high octane number. Since the
high demand cannot be supplied by the traditional
cracking processes, the catalytic isomerization of
the surplus n-butene fractions to isobutene has been
the subject of recent investigations. Especially a
∗ Corresponding author.
E-mail address: [email protected] (H. Papp).
number of 10-membered ring molecular sieves like
ferrierite [1–19], ZSM-22 [8,10,17,20–24] and the
iso-structural Theta-1 [1], ZSM-23 [25], MeAPO-11
[13,18,26–28], and SAPO-11 [8,10,13,26,28–30] have
been proven to be efficient catalysts for this reaction.
A high selectivity to isobutene and yields of isobutene
close to the thermodynamic equilibrium have been
obtained with them. H-ZSM-5 zeolites, on the other
hand, have been described as non-selective catalysts for the isomerization of n-butenes to isobutene
[1,2,4,7,8,10,11,13,14,29,31,32]. Even after isomorphous substitution of framework aluminium by iron,
leading to a reduced acid strength, the selectivity to
isobutene was lower than the selectivity obtained with
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 5 8 3 - 4
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D. Rutenbeck et al. / Applied Catalysis A: General 206 (2001) 57–66
other 10-membered ring molecular sieves [33–35].
This difference in the selectivity has been attributed
to different reaction mechanisms for the formation
of isobutene. ZSM-5 zeolites have cavities with a
diameter of ca. 0.9 nm [33] at the intersections of
the channels so that the dimerization [4,8,11,33,43]
or even trimerization [10] of n-butenes followed by
non-selective cracking of the dimers or trimers is easily possible. Molecular sieves of the structure types
FER, AEL, TON, and MTT show a higher shape
selectivity. Thus these non-selective reactions are
sterically more limited. An intramolecular isomerization [3,4,6,8,9,11,14,17,18,24,25,27,29,36–43], the
so-called monomolecular mechanism, and a pseudomonomolecular mechanism with carbenium ions
as active sites [10,12,16,44–46] have been proposed
to explain the high selectivity to isobutene obtained
with these molecular sieves.
In this paper are presented the results of a thorough investigation of the possible reaction mechanism
for the isomerization of n-butenes to isobutene on
H-ZSM-5 zeolites with a wide span of Si/Al ratio and
at varying reaction temperature.
2. Experimental
2.1. Synthesis
In Table 1, the H-ZSM-5 zeolites investigated are
listed. The crystallization of the samples synthesized
at the University Halle–Wittenberg was carried out at
448 K in a 2 l stainless-steel autoclave under moderate
Table 1
H-ZSM-5 zeolites used for investigation
Si/Al ratio
Template
Producer
17
20
22
24
28
40
60
70
160
170
Template-free
Template-free
Template-free
Template-free
TPABra
TPABr
TPABr
TPABr
TPABr
Unknown
Chemie AG Bitterfeld
VAW
University Halle–Wittenberg
University Halle–Wittenberg
University Halle–Wittenberg
University Halle–Wittenberg
Süd-Chemie
Süd-Chemie
University Halle–Wittenberg
UOP
a
TPABr: Tetrapropylammonium bromide.
stirring for 12 h. After cooling the autoclaves, the samples were taken out, filtered, washed with de-ionized
water, and dried at 393 K in air. To remove the template in case of the zeolites with Si/Al ratios from 28
to 160, the samples were calcined at 823 K in a furnace
under a flowing air stream for 4 h. The protonic form
of the resulting sodium form was prepared by heating the ammonium form, obtained by ion exchange
three times with a 1 M NH4 NO3 solution at 323 K for
16 h, at 823 K in a continuous nitrogen flow for 2 h.
The protonic form of the samples with Si/Al ratios of
17 and 170 was prepared in the same way from the
sodium form. The samples supplied by Süd-Chemie
and VAW were delivered in the protonic form.
2.2. Characterization
All samples were characterized by X-ray diffraction (XRD) performed with Cu K␣ radiation using
a Siemens D 5000 spectrometer. The diffraction patterns indicated a high crystallinity and showed only
the characteristic peaks of MFI type zeolites.
27 Al MAS NMR measurements for the determination of the framework Si/Al ratio were performed on
hydrated samples using a Bruker MSL 500 spectrometer with a sample rotation frequency of 10 kHz. Hydration was performed by keeping the samples in a
desiccator for more than 48 h over aqueous NH4 Cl.
The framework aluminium atoms in ZSM-5 zeolites
cause a narrow 27 Al MAS NMR signal at ca. 55 ppm.
Its intensity expressed in number of 27 Al atoms per
unit cell, AlF , was determined by comparing the area
of the signal with that of a well characterized ZSM-5
sample with AlF =5.3. The weight of the samples in
the rotor was taken into account for calculation. The
framework Si/Al ratio is (96/AlF )−1. The Si/Al ratio determined by 27 Al MAS NMR was confirmed by
29 Si MAS NMR for the sample with the Si/Al ratio of
28. The signal intensity of extraframework aluminium
species at ca. 0 ppm and ca. 50 ppm was less than 5%
compared to the intensity of the signal of framework
aluminium for all samples.
2.3. Catalysis
Catalytic experiments were performed at temperatures from 573 to 773 K in a continuous flow reactor
at atmospheric pressure. A mixture of 5% butene
D. Rutenbeck et al. / Applied Catalysis A: General 206 (2001) 57–66
(99.0%, AGA) in nitrogen was used as feed. The
weight hourly space velocity (WHSV) was varied
from 4 to 64 h−1 by changing the catalyst mass from
100 to 13 mg at a total flow of 60 ml/min except for
the experiments at WHSV=64 h−1 , where the total
flow was adjusted to 120 ml/min. Prior to the experiments, the catalysts were heated in situ to 723 K
in a nitrogen flow of 120 ml/min at a temperature
ramp rate of 10 K/min. A Varian 3300 gas chromatograph (GC) equipped with a flame ionization detector
(FID) and a capillary column (HP-PLOT/Al2 O3 ,
50 m×0.53 mm×0.15 ␮m) was used for product analysis. For calculation purposes, the three n-butene isomers were grouped together, since it was established
that under experimental conditions the isomerization
between 1-butene and the two 2-butene isomers is
much faster than the skeletal isomerization, and the
2-butenes are also converted to isobutene via the same
intermediate. The product distribution was calculated
from the GC peak areas divided by the number of
carbon atoms for the respective compound, since for
hydrocarbons the signal of a FID is proportional to
the number of carbon atoms [47]. The selectivity to
isobutene was determined by division of the yield of
isobutene by the conversion of n-butenes, with the
conversion defined as the percentage of the linear
butenes consumed.
59
3. Results
All samples that were synthesized using a template
exhibited an excellent stability with time on stream
(TOS). There was nearly no deactivation over the investigated reaction time of 6 h [48]. For the samples
synthesized template-free an increasing conversion of
n-butenes and an increasing yield of isobutene with
TOS was observed at 573 K. This catalytic behaviour
is yet not understood. The selectivity to isobutene,
however, remained constant. At higher temperatures
there was no difference in the catalytic behaviour
between samples synthesized with and without
template.
The conversion of n-butenes increased, whereas the
yield of isobutene and the selectivity to isobutene decreased with increasing reaction temperature on the
zeolites with Si/Al ratios up to 70. On the zeolites with
Si/Al ratios of 160 and 170 the opposite behaviour
was observed; the conversion of n-butenes decreased
and the yield of and selectivity to isobutene increased
with increasing reaction temperature. As an example
for these contrary trends in dependence on the different Si/Al ratios, the conversion of n-butenes and the
yield of isobutene is depicted in Figs. 1 and 2 as a
function of the reaction temperature for the zeolites
with Si/Al ratios of 28 and 170, respectively.
Fig. 1. Conversion of n-butenes after 10 min TOS on the zeolites with Si/Al=28 and 170 as a function of the reaction temperature
(WHSV=8 h−1 ).
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D. Rutenbeck et al. / Applied Catalysis A: General 206 (2001) 57–66
Fig. 2. Yield of isobutene after 10 min TOS on the zeolites with Si/Al=28 and 170 as a function of the reaction temperature (WHSV=8 h−1 ).
Fig. 3. Yield of the four main products at 573 K as a function of the conversion of n-butenes. The lines depict the thermodynamic limits
for the yield of isobutene.
Fig. 3 shows the yield of the four main products
obtained with all the different catalysts as a function
of the conversion of n-butenes at 573 K. In Fig. 4, the
corresponding selectivities are depicted. The straight
lines in Fig. 3 represent the thermodynamic maximum
for the yield of isobutene at the respective conversion
of n-butenes. It can be seen in Fig. 3 that the yields
increased nearly linearly with increasing conversion
of n-butenes. Only at high conversions of n-butenes
there was a decrease in the yield of propene. The
selectivities to isobutene, pentenes, and hexenes did
not change significantly with increasing conversion
of n-butenes. A selectivity value for isobutene of ca.
20% was measured independent of the conversion of
D. Rutenbeck et al. / Applied Catalysis A: General 206 (2001) 57–66
61
Fig. 4. Selectivity to the four main products at 573 K as a function of the conversion of n-butenes (symbols see Fig. 3).
n-butenes. Further reaction products at 573 K with an
amount >1 wt.% were C7 - and C8 - (they were not separated by the GC column) as well as C9 -hydrocarbons,
ethene, and different alkanes, especially isobutane and
pentanes. The C9 -hydrocarbons, ethene, and alkanes
were only detected at conversions of n-butenes >80%.
A different catalytic behaviour was observed at
higher temperatures. In Figs. 5 and 6 the yields of
and selectivities to the four main products obtained
at 773 K are plotted as a function of the conversion
of n-butenes, respectively. The values up to a conversion of n-butenes of about 60% were obtained on the
Fig. 5. Yield of the four main products at 773 K as a function of the conversion of n-butenes. The lines depict the thermodynamic limits
for the yield of isobutene.
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D. Rutenbeck et al. / Applied Catalysis A: General 206 (2001) 57–66
Fig. 6. Selectivity to the four main products at 773 K as a function of the conversion of n-butenes (symbols see Fig. 5).
samples with Si/Al ratios of 160 and 170, whereas
the other values were obtained on the samples with
Si/Al ratios up to 70. Because of experimental limitations, it was not possible to obtain the same level
of conversion of n-butenes on the zeolites with high
and low Si/Al ratio. It can be seen that at low conversions the selectivity to isobutene was much higher
than at 573 K, whereas at high conversions a lower
selectivity was observed. Extrapolated to zero conversion a selectivity value of about 90% is obtained.
At 673 K, the selectivities to isobutene were between
those measured at 573 and 773 K.
Regarding the by-products, it is striking that the
higher the reaction temperature the higher the amount
of ethene. On the other hand, the amount of hexenes and C7 - and C8 -hydrocarbons was the highest
at 573 K. The amount of alkanes (not depicted) compared at the same conversion level also decreased with
increasing reaction temperature.
4. Discussion
The low selectivity to isobutene obtained at 573 K
independent of the conversion of n-butenes and of the
Si/Al ratios of the samples shows that isobutene is
formed via the bimolecular mechanism on H-ZSM-5
zeolites at this temperature. This is also indicated by
the linearly increasing yield of the products with increasing conversion of n-butenes (Fig. 3). According
to the classification of Abbot and Wojciechowski [49],
such an increase is characteristic for primary products.
Since on the samples with Si/Al ratios up to 70, the
conversion of n-butenes increased and the yield of and
the selectivity to isobutene decreased with increasing
reaction temperature, the conclusion can be drawn that
on H-ZSM-5 zeolites with a high number of acid sites
isobutene is formed via the bimolecular mechanism
independent of the reaction temperature. On the samples with Si/Al ratios of 160 and 170 a different catalytic behaviour was observed as on those with lower
Si/Al ratios. Very high selectivities to isobutene could
be obtained with them at 773 K. These facts indicate
that there should be a change in the reaction mechanism for the formation of isobutene with increasing
reaction temperature from the bimolecular to a more
selective one on H-ZSM-5 zeolites with a low number
of acid sites.
Two mechanisms for the selective formation of
isobutene have been proposed in the literature as
pointed out in Section 1, the monomolecular and the
pseudomonomolecular one. The pseudomonomolecular mechanism requires carbenium ions as active
sites. According to Guisnet et al., these are benzylic
D. Rutenbeck et al. / Applied Catalysis A: General 206 (2001) 57–66
Table 2
Coke content of the zeolites after 6 h TOS at different temperaturesa
Si/Al ratio
17
20
28
40
60
70
160
170
a
Coke content (wt.%)
573 K
673 K
773 K
6.3
5.4
6.2
6.1
3.5
3.9
2.4
2.5
5.0
2.8
3.4
3.2
1.4
1.5
1.5
1.7
2.5
2.0
3.2
3.5
1.5
2.1
1.4
1.7
WHSV=8 h−1 .
carbenium ions at long TOS when the pores of the
molecular sieves are blocked by carbonaceous compounds [12,16,45,46]. In Table 2 the coke contents of
the samples after 6 h TOS are listed. It can be seen
that the catalysts with Si/Al ratios of 160 and 170
contained only a very low amount of coke of less than
2 wt.% after reaction at 773 K. This amount should be
too low for a blockage of the pores of H-ZSM-5 zeolites, since an amount of coke of ca. 8 wt.% is already
necessary for the blockage of the pores of ferrierites
[12,16], which have smaller pores than H-ZSM-5
zeolites [50]. The pseudomonomolecular mechanism
with benzylic carbenium ions as active sites can
therefore be excluded as an explanation for the high
selectivity to isobutene obtained with the H-ZSM-5
zeolites with a low number of acid sites at 773 K.
However, Guisnet et al. also proposed tert-butyl
carbenium ions as active sites, formed by the adsorption of isobutene on the protonic sites of the catalysts
[46]. They proposed these active sites especially for
ferrierites, since in this molecular sieve type diffusion
of isobutene might be slow due to steric constraints
so that isobutene molecules are retained inside the
pores and transformed into active sites. Because of
the larger dimension of the pores of H-ZSM-5 zeolites in comparison to ferrierites, this seems unlikely
for MFI type molecular sieves. Secondly, if mainly
tert-butyl carbenium ions were the active sites, the
selectivity to isobutene should be only slightly dependent on the temperature and the conversion of
n-butenes. It should be even more likely that a pseudomonomolecular mechanism involving tert-butyl
carbenium ions operates at low temperatures, since
the lower the temperature the slower the diffusion.
63
Thus, more isobutene molecules should be retained at
low reaction temperature and can therefore be transformed into active sites. Nevertheless, selectivities to
isobutene between ca. 20 and nearly 90% were obtained in dependence on the reaction temperature and
the conversion of n-butenes (Figs. 4 and 6). Based on
these considerations, a change from the bimolecular
to the monomolecular mechanism with increasing
temperature seems much more likely than a change
to a pseudomonomolecular one.
Assuming a change from the bimolecular to the
monomolecular mechanism, also the decreasing conversion of n-butenes and the increasing yield of
isobutene with increasing reaction temperature observed on the samples with Si/Al ratios of 160 and
170 is explainable. In Fig. 7, the yield of isobutene
obtained with the samples with Si/Al ratios of 28
and 170 at 573, 673, and 773 K after 10 min TOS,
respectively, is plotted as a function of the conversion
of n-butenes. Furthermore, the thermodynamic limit
for the yield of isobutene at each temperature in dependence on the conversion of n-butenes is shown by
straight lines. From this figure it can be seen that there
has to be a decrease in the conversion of n-butenes
when a change takes place from the bimolecular to
the monomolecular mechanism with increasing reaction temperature. Otherwise the higher yield of
isobutene formed by the monomolecular mechanism
in comparison to the bimolecular one would exceed
the thermodynamic equilibrium concentration for
isobutene. With a pseudomonomolecular mechanism,
selectively producing isobutene without or nearly
without by-products, such a drastic change in the
conversion of n-butenes and the yield of isobutene
would not be possible.
The reason for the different catalytic behaviour of
H-ZSM-5 zeolites with low and high Si/Al ratios at
high temperature is obviously related to the different
number of acid sites. At a low number of acid sites
the adsorption sites are more separated than at a high
number so that the probability of a bimolecular reaction is less. The monomolecular reaction path becomes favoured with increasing temperature because
dimerization of n-butenes is thermodynamically disfavoured at high temperature.
The formation of the different reaction products
obtained under conditions when the transformation
of n-butenes takes place mainly via the bimolecu-
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D. Rutenbeck et al. / Applied Catalysis A: General 206 (2001) 57–66
Fig. 7. Yield of n-butenes after 10 min TOS at 573, 673, and 773 K on the zeolites with Si/Al=28 (filled symbols) and 170 (open symbols)
as a function of the conversion of n-butenes (WHSV=8 h−1 ). The lines depict the thermodynamic limits for the yield of isobutene.
lar mechanism (low temperature, high number of
acid sites) can be explained by the scheme shown in
Fig. 8. First, n-butenes dimerize to C8 -units. These are
mainly cracked into propene and pentenes and to a minor part into isobutene as well as hexenes and ethene.
Furthermore, the C8 -units can react to C12 -units,
oligomers, and aromatics. Oligomers and aromatics
are the precursors of coke. Aromatics were observed
Fig. 8. Reaction scheme for the transformation of n-butenes on
H-ZSM-5 zeolites with a high number of acid sites and at relatively
low temperature.
by Houzvicka et al. [31] when they investigated the
skeletal isomerization of n-butenes to isobutene on
H-ZSM-5 zeolites. Unfortunately, aromatics could not
be detected by the GC column we used for our work.
It is likely that a part of the products formed by
cracking of the C8 -units is transformed into nonenes
and heptenes in consecutive reactions. The C12 -units
can be cracked into hexenes, pentenes and heptenes as
well as propene and nonenes. Furthermore, they can
also react to oligomers, aromatics, and coke like the
C8 -units. The transformation of alkenes into alkanes
is not depicted in Fig. 8, since otherwise the scheme
would be overcrowded. The formation of alkanes can
be explained by hydrogen transfer to the respective
alkenes during the formation of aromatics and coke
[8,51].
The scheme in Fig. 9 is proposed in order to explain the formation of the different products observed
under reaction conditions when the transformation
of n-butenes takes place mainly via the monomolecular mechanism (high temperature combined with
a low number of acid sites). The main reaction is
the monomolecular isomerization of n-butenes to
isobutene. Besides, some n-butene molecules dimerize to C8 -units. At high conversions of n-butenes,
co-dimerization with isobutene molecules and
dimerization of the last ones also takes place. The
D. Rutenbeck et al. / Applied Catalysis A: General 206 (2001) 57–66
Fig. 9. Reaction scheme for the transformation of n-butenes on
H-ZSM-5 zeolites with a low number of acid sites at high temperature.
consecutive reactions of the C8 -units are the same as
mentioned above, except that probably no C12 -units
are involved. Since the amount of ethene and propene
detected was much higher than the amount of hexenes
and pentenes, respectively, the last ones are likely to
be partly transformed into oligomers, aromatics, and
coke, too. Another possibility would be the cracking
of hexenes into propene.
5. Conclusion
On H-ZSM-5 zeolites with a high number of acid
sites (low Si/Al ratio) the isomerization of n-butenes
to isobutene proceeds via the bimolecular mechanism
independent of the reaction temperature. On H-ZSM-5
zeolites with a low number of acid sites the reaction mechanism changes from the bimolecular to the
monomolecular one with increasing temperature.
Acknowledgements
The work has been supported by DFG within Sonderforschungsbereich 294.
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